In many polymerization processes for the production of normally solid olefin polymers such as polypropylene, polyethylene and copolymers thereof, a stream is formed which is a slurry of particulate polymer solids suspended in a liquid medium which is usually the reaction diluent. Typically these polymerization processes employ a relatively high pressure continuous reactor, such as a loop reactor, which is operated at a pressure in a range of about 600 to 650 psig and may have a downwardly depending settling leg to facilitate product removal. In such processes a slurry of produced solid polymer and diluent is discharged into a separation chamber where e.g., the slurry is flashed to a low pressure such as about 20 psia. Further information and details of the polymerization process, including examples of suitable reaction conditions as well as control schemes for other important variables, such as solids concentration, production rate, etc., can be found in U.S. Pat. No. 3,998,995 of Buss et al and U.S. Pat. No. 3,257,363 of Miller et al, the disclosures of which are incorporated herein by reference.
A problem encountered in controlling removal of polymerization effluent from a loop reactor is that the solids must be maintained suspended in a diluent to prevent solidification. Otherwise the control valve in the effluent line becomes plugged with solid polymer. Since the primary valve controlling effluent removal may become plugged with solid polymer and be inoperative, a removal system that includes a backup or secondary removal line that automatically operates when the primary line is unavailable is highly desirable.
Accordingly, it is an object of this invention to withdraw reaction effluent from a polymerization reactor in a safe and efficient manner.
Another object of this invention is to operate a primary reaction effluent line in a manner that maintains stable reactor conditions.
A further objective is to operate a backup reaction effluent line in a manner that minimizes the likelihood of complete plugging of the removal system with solid polymer.